I found a workaround by running Disconnect (another VPN app) that seems to be able to close all other connections and route all traffic through the Network Extension module. Otherwise Mail and Safari traffic is not routed through AdGuard. But the downside is I'm unable to de-select a browser from AdGuard filtering. It would still be filtered. This is not the behavior it previously was where Mail DNS would be filtered but Opera (de-selected from filtering) is not being filtered for DNS.